Yessss, let me go on about my new blorbos, yesssss. More about sleep in general than the Sleep spell, but it is canon setting and not an AU!
Zahra doesn't tend to have nightmares. She hasn't since she made her pact with Sirius, and meditating under the moon before she sleeps has always soothed her enough to fend off many of the worst memories of her childhood.
Besides. Her father was a shit - she knows this, and she knows his justifications. It's almost easier, really, knowing it was all due to him and his own bullshit, that it was never due to her - it's not like she asked to be a tiefling, though she wouldn't give it up for anything - and truly, if anyone has the evil inclination of the hells, it was her father and not her.
Still - she's not without them completely, they're just rare. And, moreover, they're only nightmares - not the kind of creeping bad dream that echoes painfully into the waking world. Generally she wakes and remembers reality and is fine.
He sleeps fitfully - it hadn't bothered him to take any watch on their first job together; he woke easily no matter the time, but always seemed tired in addition to his grumpiness. Spending more time with him only makes it more obvious: some mornings Kash joins her for breakfast with deep bags beneath his eyes, the expression of someone who's barely slept at all.
She knows better than to pry. She knows more than most people do about Kash's history and she thinks that's more than enough to guess. Especially given his behaviour: it's clear his experiences left him deeply and lastingly scarred and where she's worked to process it and move forwards - to be free just as her Patron is - she doesn't think Kash has even tried.
Then again, given the seeming omnipresence he was taught to believe of his goddess, she's not that surprised. What point is there trying to heal when one will only be hurt again?
Still, it surprises her, as she and Kash get closer, how easily he sleeps in her arms. She thinks it's an extension of his general ease with her, the tacit reassurance born of the fact she saved his life: he knows he's safe with her, because she has proven she will fight for him. He knows he's safe with her, because she's gone out of her way to ensure his wellbeing.
Still - some nights are rougher than others. For all her lack of nightmares, she does tend to sleep lightly - a childhood under her father's watchful eye ensured it - and so she stirs easily those few times Kash's sleep is disturbed.
"I'm here," she murmurs, in that space between waking and sleeping, when she feels Kash stir. Her hand finds his scarred arm, her tail, where it's coiled over her hip and his, reaches for his wrist. His calluses graze against her skin, flexing tight before relaxing at her warmth. "I'm here," she repeats and nuzzles his neck. "You're safe, Kash."
And, without waking further, he settles easily back into sleep.
She can't prevent his nightmares, not anymore than she can prevent her own, but she can ensure he never has to wake from them alone.
In her arms, Kash sleeps, and sleeps peacefully.